1. Field of the Invention
The present invention relates to an ultrasonic diagnostic system for imaging according to an ultrasonic Doppler signal obtained from a subject and a method of processing ultrasonic image data.
2. Description of the Related Art
Ultrasonic diagnostic systems radiate ultrasonic pulses generated from piezoelectric vibrators incorporated in an ultrasonic probe into a subject, receive reflected ultrasonic waves generated by the difference in acoustic impedance of the tissue of the subject with the piezoelectric vibrators, and display it on a monitor. The diagnostic technique is widely used for diagnosing the functions and shapes of the various organs of living bodies, because it allows easy observation of a real-time two-dimensional image by a simple operation of bringing the ultrasonic probe into contact with the surface of the body.
The ultrasonic diagnostic technique of acquiring living-body information from the tissues of living bodies or reflected waves from blood cells has made a remarkable progress owing to two great technological developments, an ultrasonic pulse reflection method and an ultrasonic Doppler method; a B-mode image and a color Doppler image obtained by using the above techniques are absolutely essential in the present ultrasonic diagnostic imaging.
The color Doppler method is a method whereby a specified cross section of a living body is scanned with ultrasonic pulses, wherein when a moving reflector such as blood (blood cells) is irradiated with the ultrasonic waves, a Doppler frequency shift generated in correspondence with the velocity of the reflector (blood flow rate) is imaged. The color Doppler method was at first used to image the high blood flow in a cardinal cavity, but today it has been applicable to imaging of extremely low blood flow such as the blood flow of the tissues of abdominal organs.
In order to enhance the diagnostic ability of the color Doppler method, it is necessary to have, firstly, high measuring accuracy (particularly, low-flow-rate detectability), secondly, time resolution (real-time characteristic), and thirdly, spatial resolution.
When ultrasonic pulses are radiated to a moving reflector to measure the velocity of the reflector from the Doppler frequency shift of reflected waves, it is necessary to repeat transmission and reception of ultrasonic waves to/from the reflector multiple times (L times) at a rate interval Tr, thereby measuring the moving speed of the reflector from a series of acquired reflected waves. In this case, the detectability (measuring range lower limit of a flow rate) Vmin of a low-speed reflector depends on the frequency resolution Δfd of frequency analysis for a series of reflected waves acquired by the n times of ultrasonic transmission and reception. The frequency resolution Δfd is expressed as equation (1)Δfd=fr/L  (1)where fr (fr=1/Tr) is an ultrasonic transmission reception repetition frequency (rate frequency).
In other words, in order to increase the low-flow-rate detectability, the first requirement in the color Doppler method, it is necessary to delay the rate frequency fr or increase the ultrasonic transmission and reception repetition times L in a predetermined direction.
The real-time characteristic, the second requirement, is determined depending on the number of display images (frame frequency) Fn per unit time. The frame frequency Fn is expressed as equation (2)Fn=fr/L/M=Δfd/M  (2)where M is the total number of scanning lines (raster) necessary to construct one piece of color Doppler image data. In order to improve the real-time characteristic, the transmission and reception number L or the total number M of scanning lines must be set small.
In order to enhance the spatial resolution, the third requirement, it is necessary to increase the total number M of scanning lines. Since the frame frequency Fn, the detectability Vmin, and the spatial resolution are mutually contradictory and so it is difficult to satisfy them at the same time. Accordingly, for measuring blood flow in a circulatory region, the frame frequency has been regarded as important and, for measuring blood flow in an abdominal region and peripheral regions, low-flow-rate detectability has been regarded as important, in both of which the spatial resolution has been disregarded.
For the decline in spatial resolution, temporal or spatial filtering is performed by using a low-pass filter or a median filter (median extraction filter) to smooth the boundary of blood vessels or blood-flow distribution or to reduce image defect (so-called a black hole pattern) due to the interference of ultrasonic waves etc. However, the application of the conventional filtering method makes it difficult to display the boundary of a blood flow pattern with the smoothing of the boundary and the decrease of the black hole pattern, resulting in a decrease in diagnostic ability.
To overcome the above problems, for example, JP-A-2000-262520 proposes a median filter that has the function of determining whether the value of a specified pixel (target pixel) on color Doppler image data is a singular value such as black hole. In this method, the difference between the value of a specified pixel (target pixel) of Doppler image data and the value of its peripheral pixel (reference pixel) is calculated, wherein when the difference is smaller than a predetermined threshold value, the value of the reference pixel is replaced with the value of the target pixel, then the median value is extracted from the values of the reference pixel and the target pixel, and the median value is substituted by the value of the target pixel.
Although the above method allows a black hole pattern to be decreased without reducing spatial resolution, it is difficult to display a mosaic pattern caused by a turbulent flow or a backward flow generated in a narrow part of a blood vessel (hereinafter, referred to as a turbulent mosaic pattern).
Blood flows at a high rate in a narrow part of a blood vessel to generate a turbulent flow, so that a blood flow moving close to the ultrasonic probe and a blood flow moving away from the probe are mixed. Moreover, at the high blood flow rate, the Doppler frequency exceeds a Nyquist frequency determined by a sampling frequency to generate a turning back phenomenon.
Although the turbulent mosaic pattern generated by those phenomena is very important information for an ultrasonic diagnosis of a narrow part, the foregoing method determines the pixels in the turbulent mosaic pattern to be singular points, so that the clearness or brightness of the image is decreased, thus making it difficult to observe.
Furthermore, when the reference pixel and the target pixel are composed of plus pixel values and minus pixel values of approximately the same number as the plus pixel values, the median value approaches zero, and so the turbulent mosaic pattern cannot be displayed.